The Open System Interconnection (OSI) reference model uses seven different layers to define system interconnections. The first layer of the OSI model is the physical layer, which defines the basic hardware functions through which two systems can physically communicate raw bits of information. The physical layer includes hardware to transmit electromagnetic signals including, for example, electrical, wireless, and optical signals.
The third layer of the OSI model is the network layer, which is responsible for connection functions, addressing, and routing of information from a source to a destination. The third layer of the OSI model operates using the physical interconnections defined at the physical layer. In a system where the network layer is an Internet Protocol (IP) network layer, a network is defined by a plurality of interconnected nodes or routers that each have an IP address. Packets of data are then transmitted over router-to-router links between different source-destination pairs. Not every pair of nodes or routers is directly connected and some source-destination pairs must send packets over a series of direct node-to-node or router-router connections to exchange traffic.
In an IP-over-wavelength division multiplexing (WDM) architecture, the communication links or IP links (network layer) between router pairs are established using wavelengths carried over optical fibers (physical layer). Using WDM, each optical fiber in the optical network can carry multiple wavelengths and, thus, multiple IP links. In particular, a given physical link in the optical network may carry wavelengths that connect multiple different router pairs.